1. Field of the Invention
The present invention relates to a diversity receiver and a method that are suitable for an Orthogonal-Frequency-Division-Multiplexing receiver or the like.
2. Description of the Prior Art
In recent years, in the field of mobile communication, the Orthogonal-Frequency-Division-Multiplexing (OFDM) method has been employed from time to time, for example, because of its superior properties against multipath. Because, also in terrestrial digital broadcasting, it is desired that the high-definition broadcast intended for stationary reception be able to be received also by mobile bodies such as vehicles, the OFDM method is employed.
In addition, in terrestrial digital broadcasting utilizing the OFDM method, a directivity control technique utilizing a plurality of antennas may be employed as a technique for raising the reception quality of mobile reception. In the foregoing diversity-method OFDM receiver, a plurality of branches as reception circuits are incorporated, and each branch OFDM-demodulates a reception signal so as to obtain complex symbol data.
For example, in the case of 4-channel diversity system, branches obtain four respective complex symbol data from the output terminals of four antennas. Then, by synthesizing the outputs from the branches, the complex symbol data based on a reception signal is obtained. The complex symbol data is error-corrected in an error correction circuit and through demapping processing corresponding to the constellation of the modulation method, converted into the original information signal.
Meanwhile, when four complex symbol data are synthesized, the synthesis proportions are determined in accordance with the respective reliability levels of the branch outputs. For example, in Japanese Patent Laid-Open No. 11-150497, an example is disclosed in which branch outputs are synthesized at synthesis proportions in accordance with the respective reliability levels of the branch outputs.
However, in determination of the reliability levels of the respective branch outputs, an error may occur that causes a branch output of low reliability to be synthesized at a high synthesis proportion, whereby the error rate of the complex symbol data may be enhanced.
For instance, the reliability levels of respective branch outputs may be determined based on the S/N ratios. The S/N ratio is obtained based on the distance between the position, of the complex symbol data from a branch, in the constellation and the symbol reference point. When, e.g., due to intrusion of large noise, the position, of the complex symbol data from a branch, in the constellation becomes close to a symbol reference point that is different from the symbol reference point with respect to which the complex symbol data should originally be demapped, the complex symbol data may be demapped into an erroneous code and the branch output may be determined to be a relatively high S/N ratio. In this case, the complex symbol data including an error is synthesized at a high synthesis proportion. As a result, the probability that the synthesized complex symbol data has an error is raised, whereby the quality of the reproduced information signal is deteriorated.